I know, it's a shame and I must be the xth member to post a message here about this issue, but no advice proved to work on my machine.

The setup is a brand new Debian 4.0.
I followed the HOWTO 'Virtual Users And Domains With Postfix, Courier And MySQL - ubuntu'.
During the installation no errors occured, so it cannot be totally screwed.

What is the (my) problem?
It is not possible to send/receive emails via a client (thunderbird in my case).

# sasldb -- use the local sasldb database file
# ldap -- use LDAP (configuration is in /etc/saslauthd.conf)
#
# Only one option may be used at a time. See the saslauthd man page
# for more information.
#
# Example: MECHANISMS="pam"
MECHANISMS="pam"
# Additional options for this mechanism. (default: none)
# See the saslauthd man page for information about mech-specific options.
MECH_OPTIONS=""
# How many saslauthd processes should we run? (default: 5)
# A value of 0 will fork a new process for each connection.
THREADS=5
# Other options (default: -c)
# See the saslauthd man page for information about these options.
#
# Example for postfix users: "-c -m /var/spool/postfix/var/run/saslauthd"
# Note: See /usr/share/doc/sasl2-bin/README.Debian
#OPTIONS="-c"
OPTIONS="-m /var/spool/postfix/var/run/saslauthd -r -c"
PIDFILE="/var/spool/postfix/var/run/${NAME}/saslauthd.pid"